This invention relates to fuel cells arranged in a fuel cell stack and, in particular, to a fuel cell stack design and method configured to enhance overall fuel utilization and control temperature distribution in the stack and thereby provide an increased service life for the stack.
A fuel cell is a device which directly converts chemical energy stored in hydrocarbon fuel into electrical energy by means of an electrochemical reaction. Generally, a fuel cell comprises an anode and a cathode separated by a member which serves itself to conduct electrically charged ions or is adapted to hold an electrolyte which conducts electrically charged ions. In order to produce a useful power level, a number of individual fuel cells are stacked in series with an electrically conductive separator plate separating the cells.
Before undergoing the electrochemical reaction in the fuel cell, hydrocarbon fuels such as methane, coal gas, etc. are typically reformed to produce hydrogen for use in the anode of the fuel cell. In internally reforming fuel cells, a steam reforming catalyst is placed within the fuel cell stack to allow direct use of hydrocarbon fuels without the need for expensive and complex reforming equipment. In addition, the endothermic reforming reaction can be used advantageously to help cool the fuel cell stack.
Internally reforming fuel cells employ direct internal reforming and indirect internal reforming. Direct internal reforming is accomplished by placing the reforming catalyst within the active anode compartment. Direct internal reforming thus directly exposes the catalyst to the electrolyte of the fuel cell, which can lead to deactivation of the catalyst and an eventual degradation of the fuel cell's performance. Improvements have been made to the direct internal reforming technique to reduce electrolyte contamination, but these improvements are accompanied by high costs due to the complexity of the fuel cell design, special materials requirements and a reduction in the effectiveness of the reforming catalyst.
The second reforming technique, indirect internal reforming, is accomplished by placing the reforming catalyst in an isolated chamber within the fuel cell stack and routing the reformed gas from this chamber into the anode compartment of the fuel cell. With this technique, the need for separate ducting systems raises the cost of the fuel cell stack and also makes the system susceptible to fuel leaks.
The current state of the art uses a hybrid assembly in which the fuel cell stack has both direct and indirect internal reforming and in which external manifolds are used for enclosing and directing the flow of fuel and oxidant gases into the stack. U.S. Pat. No. 6,200,696 and U.S. Patent Application Publication No. 2006/0123705, assigned to the same assignee hereof, disclose examples of such hybrid assemblies. As disclosed in the '696 patent and the 2006/0123705 publication, the hybrid assembly includes one or more fuel reformers for indirect internal reforming of input fuel gas, which receive the input fuel gas and convey it in a U-shaped path while reforming the fuel therein. The assembly of the '696 patent and the 2006/0123705 publication also includes a fuel-turn manifold for redirecting reformed gas outputted by the indirect internal reformers to the anode compartments for further reforming through direct internal reforming and electrochemical conversion. In these assemblies, both the U-shaped flow path in the reformer and the flow through the anode compartments of the fuel cells is in cross-flow, or perpendicular to, the oxidant gas passing through the stack.
Due to the nature of the fuel flow within the fuel reformers, such hybrid assemblies are sometimes susceptible to non-uniformity in their current density distribution and to temperature gradients near the gas exits of the stack. These effects occur as the stack ages and as the catalyst within the stack plates, the Direct Internal Reforming (DIR) catalyst, is deactivated over the course of the service life of the stack. As a result, thermal instability within the stack may occur and may cause non-optimized fuel utilization in the production of electricity. This is especially true given the maximum allowable temperature at which the stack is designed to operate.
It is therefore an object of the present invention to further improve fuel cell stack design and methodology so as to create a fuel flow arrangement which increases the fuel conversion efficiency of the stack.
It is also an object of the present invention to provide a fuel cell stack design and methodology which promotes cooling so as to realize a more uniform temperature distribution, thus increasing the overall efficiency of the fuel cell operation and electricity production and extending the operating life of the stack.